I’m replacing the unshielded power cord on a microphone
preamp. Besides having an insulated ground wire there is
a raw wire lead. That raw lead is attached to the plug
ground, of course, but in addition to the foil wire wrap and
in turn to the stranded wire shielding. What is the purpose
of the raw wire lead and what should be done with it? I was
under the impression that for maximum RF/EMF rejection
the coax shield/foil only drained into the common ground of
the plug, not to the equipment itself.

"Powell" wrote in message
...
I'm replacing the unshielded power cord on a microphone
preamp.

unshielded?

Besides having an insulated ground wire there is
a raw wire lead. That raw lead is attached to the plug
ground, of course, but in addition to the foil wire wrap and
in turn to the stranded wire shielding. What is the purpose
of the raw wire lead and what should be done with it? I was
under the impression that for maximum RF/EMF rejection
the coax shield/foil only drained into the common ground of
the plug, not to the equipment itself.

I'll speculate as its kind of hard to know for sure without knowing the
details of the amp.

I think the amp chassis also serves as EMI shield. Its gotta be tied to
earth ground by direct a path as possible. Then one end of the cable shield
(and wire) goes to earth ground and the other end connects to chassis to
provide an earth ground for it which is different than electrical ground
(although in most house wiring they get tied together at the box).
Unfortunately stray ground current returning via chasis ground (through
cable shield which is too high impedance) can create a chassis potential
caused by a voltage drop across the cable shield. The bare wire is just
there to reduce the overall shield path impedance and minimize the voltage
drop that can be created by stray ground currents returning via chassis
ground vs neutral. I think U/L allows some percentage (like 5 or 15, cant
remember exactly) of total current to return via chassis ground.

ScottW

"Powell" said:

Im replacing the unshielded power cord on a microphone
preamp. Besides having an insulated ground wire there is
a raw wire lead. That raw lead is attached to the plug
ground, of course, but in addition to the foil wire wrap and
in turn to the stranded wire shielding. What is the purpose
of the raw wire lead and what should be done with it? I was
under the impression that for maximum RF/EMF rejection
the coax shield/foil only drained into the common ground of
the plug, not to the equipment itself.

The shielded power cords I saw were mostly wired such that a separate
ground lead inside the shield was connected to chassis on the eq.
side, and to ground at the mains side.
The shield was connected only at the plug's side to ground, and was
left open at the equipment side.

This way, one avoids ground loops in the cord.

The bare wire in your cord would render such an arrangement
impossible, since it will be able to make contact to the shielding.

--
Sander de Waal
" SOA of a KT88? Sufficient. "

"Powell" wrote in message


I'm replacing the unshielded power cord on a microphone
preamp.

Elsewhere you talk about foil wrap and standed wire shielding. Please
clarify - is this cable shielded or not?

Besides having an insulated ground wire there is
a raw wire lead.

That would be the drain wire for the foil shield.

That raw lead is attached to the plug ground, of course,

Not a given.

but in addition to the foil wire wrap and in turn to the stranded wire
shielding.

That's what drain wires do.

What is the purpose of the raw wire lead and what should be done with it?

Its the thing you use to make an electrical connection to the foil wrap.

I was under the impression that for maximum RF/EMF rejection
the coax shield/foil only drained into the common ground of
the plug, not to the equipment itself.

The drain wire can be used either way, or both.

Consider that the world is full of mic preamps that have unshielded power
cords and work just fine. The shielding serves very little purpose in many
environments where the power wiring in the building is unshielded. Any
interference that the shelded cord may wish to protect the power wiring from
can easily come in via the unshielded wiring in the building.

"Powell" wrote in message
...
I'm replacing the unshielded power cord on a microphone
preamp. Besides having an insulated ground wire there is
a raw wire lead. That raw lead is attached to the plug
ground, of course, but in addition to the foil wire wrap and
in turn to the stranded wire shielding. What is the purpose
of the raw wire lead and what should be done with it? I was
under the impression that for maximum RF/EMF rejection
the coax shield/foil only drained into the common ground of
the plug, not to the equipment itself.


From the confusion expressed by everyone else, I'm confused by your
question. I will attempt to answer it in a way that covers all the bases.

First of all, make sure the preamp is earthed. One way or another, there
must be a DC ground betwen the mic preamp and power socket ground. With
hi-fi equipment, some people play around with ground lifts, but this must
NEVER BE DONE with a mic preamp. If the chassis is not properly grounded,
individuals who handle microphones and inadvertently ground themselves could
be lethally exposed.

IF there is a second wire that appears to have a shielding function, but is
not part of the DC ground setup, you have the opportunity to connect it any
way you want. However, your objective should be to maintain this additional
shield at the potential of the mic preamp box, which is it's own Faraday
cage reference.

THEREFORE, from the standpoint of hum reduction, connect this second wire,
if it exists, only at the preamp end of the cord.

"Powell" wrote

Gentlemen, thank you for your responses. Just to add
more information. Here is the subject replacement cord.
http://www.volexpowercords.com/power...umber=&userid=

The ground plug (outlet side) is connected to an
insulated lead wire. This is conventional and expected.
But in addition there is a raw lead wire which is
connected to the ground plug. And finally, the Duofoil
wire wrapping and around that the braid wire shielding
are connected to the ground plug, too. In total there are
four electrical pathways to the ground on the plug end.

There seems to be two alternatives. Tie the raw and
insulated ground wire leads together and attach the
combined wires to the preamp housing (ground point).
The alternative is to only attach the insulated ground
wire to the equipment ground point. But then again
maybe I'm missing something.

Which is the better approach to reject EMI radiation to
other cables and isolation of this one?

"Arny Krueger" wrote

I'm replacing the unshielded power cord on a microphone
preamp.

Elsewhere you talk about foil wrap and standed wire shielding.
Please clarify - is this cable shielded or not?

See product description on my 2nd post.


Besides having an insulated ground wire there is
a raw wire lead.

That would be the drain wire for the foil shield.

And the braid wire shielding, too.


That raw lead is attached to the plug ground, of course,

Not a given.

No, it is based on meter reading.


but in addition to the foil wire wrap and in turn to the
stranded wire shielding.

That's what drain wires do.

Why are four electrical pathways needed?


What is the purpose of the raw wire lead and what
should be done with it?

Its the thing you use to make an electrical connection
to the foil wrap.

That's clear as mud.


I was under the impression that for maximum RF/EMF
rejection the coax shield/foil only drained into the common
ground of the plug, not to the equipment itself.

The drain wire can be used either way, or both.

Unlikely, otherwise why would the manufacture design
the cable this way giving one choice?


Consider that the world is full of mic preamps that have
unshielded power cords and work just fine.

OSAF


The shielding serves very little purpose in many
environments where the power wiring in the building is
unshielded. Any interference that the shelded cord may
wish to protect the power wiring from can easily come in
via the unshielded wiring in the building.

Which is why the amp is connected to a power conditioner
with RF/EMI filtering with a isolated transformer.
Unfortunately there are several other cables together with
this one like power, analog, digital, ect.

"Robert Morein" wrote

From the confusion expressed by everyone else, I'm
confused by your question. I will attempt to answer it
in a way that covers all the bases.

First of all, make sure the preamp is earthed. One way
or another, there must be a DC ground betwen the mic
preamp and power socket ground. With hi-fi equipment,
some people play around with ground lifts, but this
must NEVER BE DONE with a mic preamp. If the
chassis is not properly grounded, individuals who
handle microphones and inadvertently ground
themselves could be lethally exposed.

Understood. The old cable had a insulated ground
wire attached to the amp case (ground point).


IF there is a second wire that appears to have a
shielding function, but is not part of the DC ground
setup,

Oh, but it is a DC ground, too.


THEREFORE, from the standpoint of hum reduction,
connect this second wire, if it exists, only at the preamp
end of the cord.

That's not an option. If the raw wire is connected to
amp it is in direct connection (pathway) to ground on
plug. Using a meter the insulated ground and raw
ground wire measure to the thousandths as having
the same electrical conductivity to the ground on the
plug end.

"Powell" said:


"Powell" wrote

Gentlemen, thank you for your responses. Just to add
more information. Here is the subject replacement cord.
http://www.volexpowercords.com/power...umber=&userid=

The ground plug (outlet side) is connected to an
insulated lead wire. This is conventional and expected.
But in addition there is a raw lead wire which is
connected to the ground plug. And finally, the Duofoil
wire wrapping and around that the braid wire shielding
are connected to the ground plug, too. In total there are
four electrical pathways to the ground on the plug end.

There seems to be two alternatives. Tie the raw and
insulated ground wire leads together and attach the
combined wires to the preamp housing (ground point).
The alternative is to only attach the insulated ground
wire to the equipment ground point. But then again
maybe I'm missing something.

Looks to me that the raw lead is the connection to the shield.
The "real" ground wire is insulated, and should be connected at all
times.
The shield connection could be an option, assuming the raw lead +
shield are already connected at the mains plug side..
Just try it and listen or measure with an oscilloscope which situation
is best.
Usually, holding a cellular phone near the device or cord should tell
you what's best.

--
Sander de Waal
" SOA of a KT88? Sufficient. "